The overall objectives of this project are to define the heterogeneity of cells and cell functions involved in the regulation of immune responses. During the next project year the major goal will be to study the functions of accessory cells in several in vitro systems. A second area of investigation has been to develop new methods for studying T cell heterogeneity. BIBLIOGRAPHIC REFERENCES: Shiigi, Stanley M. and Mishell, R.I., Sera and the in vitro induction of immune responses. I. Bacterial contamination and the generation of good fetal bovine sera. J. Immunol., 115: 741, l975. Shiigi, Stanley M., Fujikawa, S. and Mishell, Robert I., Sera and the in vitro induction of immune responses. II. Inhibitory effects of newborn calf sera on primary humoral responses. J. Immunol. 115:745, l975.